Coast / Estuary

Coastal systems are among the most dynamic physical systems on earth and are subject to a large variety of forces. The morphodynamic changes occurring to coastlines worldwide are of great interest and importance. These changes occur as a result of the erosion of sediments, its subsequent transport as bed load or suspended load, and eventual deposition. 
 
Estuaries are partly enclosed water bodies that have an open connection to the coast. Estuaries generally have one or more branching channels, intertidal mudflats and/or salt marshes. Intertidal areas are of high ecological importance and trap sediments (sands, silts, clays and organic matter).
Within the Delft3D modelling package a large variation of coastal and estuarine physical and chemical processes can be simulated. These include waves, tidal propagation, wind- or wave-induced water level setup, flow induced by salinity or temperature gradients, sand and mud transport, water quality and changing bathymetry (morphology). Delft3D can also be used operationally e.g. storm, surge and algal bloom forecasting. 
 
On this discussion page you can post questions, research discussions or just share your experience about modelling coastal and/or estuarine systems with Delft3D FM. 
 

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Sub groups
D-Flow Flexible Mesh
DELWAQ
Cohesive sediments & muddy systems

 


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RE: Estuary model - unstable downstream boundary

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Hello all,

I'm trying to setup a model of an estuary and I'm running into several problems.

As boundary conditions I have a constant discharge upstream and a time series of a water gauge downstream.

In the first few tries the model wouldn't run stable at all. I had to lower the time step to 0.5 min and use a very coarse grid with approximately 20m * 10m cells. The model crashes with the error "Velocity change too high" if not for the coarse grid and low time step. I checked the Courant number which shows that I should be able too run the model even with longer time steps.

The model that runs produces strange results at the downstream boundary as seen in the attachment (ds.fig). These maybe the reason why it crashes.

I tried a lot different time steps and grid resolutions but I can't seem to get good results. Any input would be appreciated.

My aim is to produce a model with a resolution of about 2m x 2m in a dockyard that lies near the downstream boundary. Any input on how i can achieve that would be appreciated as well.


Thanks in advance

Jan
Attachments: Hunte.bct (2,137.2k), Hunte.dep (167.5k), Spline10x10.grd (539.1k), ds.fig (194.2k)
RE: Estuary model - unstable downstream boundary
initial conditions instability timestep
Answer (Unmark)
10/6/17 12:23 AMas a reply to Jan Tiede.
Hi Jan,

The required time-step to achieve accuracy and stability can be calculated using the CFL number. I suggest reading section "4.5.3 Time Frame" of the manual for an overview. For example if I assume cells of dimension = 10m and depth = 10m then the timestep required to achieve accuracy would be ~10s. This suggests your timestep (30s) may still be too large? If you reduce your grid size to 2m you will also need to reduce your timestep.

As well as the timestep you may want to give some attention to the initial conditions. If you are cold starting the model from a zero velocity constant water level (or similar) you may need to do an initial short duration simulation with a very short timestep and then use the final outputs from this simulation as an initial condition for the main simulation with a longer timestep. This helps ensure that the model has a smooth start. The "Smoothing time" is also an important parameter to help get the model started if you don't have good initial conditions.

Hope that helps,
Richard
RE: Estuary model - unstable downstream boundary
Answer (Unmark)
10/11/17 2:13 AMas a reply to Richard Measures.
Hi Richard,

Thanks for your quick response.

How would I use the results from the warm up model? Do I use the computed water level as input for my initial conditions in the actual model?

Cheers

Jan
RE: Estuary model - unstable downstream boundary
initial conditions trim2ini
Answer (Unmark)
10/11/17 5:29 AMas a reply to Jan Tiede.
Hi Jan,

You can use the outputs from one simulation as initial conditions to another in 3 ways:
  1. Use a restart file as initial conditions
  2. Use the map file (*trim.dat) as initial conditions
  3. Generate a *.ini file from the trim file using the matlab open earth tool "delft3d_trim2ini".
  4. Generate a *.ini file manually by combining grids of water level and velocity exported from quickplot.

The GUI has options for how you want to specify the initial condition (constant water level/restart file/map file/ini file).
I personally prefer option 3 as it gives complete control and transparency over the initial conditions (for example you can pick a specific time-step to generate initial conditions from) but is streamlined and easy to do.

Cheers,
Richard